Device for improving the tone qualities of loudspeakers



April 23, 1940- E J. H. BUSSARD DEVICE FOR IMPROVING THE TONE QUALITIES 0F LOUDSPEAKERS Filed Aug. 17, 1956 INVENTOR.

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Patented Apr. 23, 1940 UNITED STATES PATENT OFFICE DEVICE FOR IMPROVING THE TONE QUALI- TIES OF LO-UDSPEAKERS Application August 17, 1936, Serial No. 96,370

4 Claims.

I shall describe my invention in connection with loudspeakers in radio cabinets, it being understood that this application of the invention is not a limitation on it, but that there are many other uses for loudspeakers involving the same problems and susceptible of the same solutions as I shall set forth herein.

It has been the practice in the past to mount a loudspeaker to a mounting board, the loud 1' speaker being behind the board, and the board being cut away circularly to uncover something less than the full diameter of the cone. The board has acted as a batlle. Usually it is a separate board held to the cabinet front by screws; but this has been a matter of convenience largely in the construction of cabinets. Loudspeakers could have been mounted directly to the front face of the cabinet if that front face were strong enough.

I have found that loudspeakers so mounted are subject to a number of disadvantages. The vibration. of the speaker is frequently transmitted through the cabinet to the radio set, producing microphonism. The high audio frequencies sent out primarily from the central portion of the cone of the speaker, have a distinct beam characteristic, so that not only is it possible by ear to tell the definite location of the sound producing contrivance, but also the reproduction sounds differently in different parts of the room, and there tend to be distinct nodal characteristics in the space of the room. There is, of course, always the problem of specific resonances in the loudspeaker. Finally there are what are known as wolf tones arising in various ways in the speaker. One serious way in which these tones arise, I have found, is through the vibration of the so-called skiver or edge portion of the cone. This is especially important today where the edge portion is a part formed integral with and of the same characteristics as the cone itself, turned into a horizontal plane and usually having one or more corrugations between the edge of the coneshaped part and the clamping ring. These edge portions of the speaker cone tend to vibrate at stray frequencies, often having no relationship to the frequencies being reproduced by the speaker. The fundamental objects of my invention are the provision of correctives for all of these difficulties by means which are simple and inexpensive, and the provision of an acoustic loading for its own sake where the nature of the output circuit is such as to make it advisable. I accomplish these objects and more specific ones which will be set forth hereinafter, or will be apparent to one skilled in the art upon reading these specifications, by that certain construction and arrangement of parts, of which I shall now describe the aforesaid exemplary embodiment. For a clearer understanding of the invention, reference is now made to the drawing, wherein:

Fig. l is a sectional view of an exemplary radio cabinet showing a loudspeaker mounted therein.

Fig. 2 is a plan view of the main baflle and of the loudspeaker mounting member which I shall hereinafter describe.

Fig. 3 is a sectional view taken along the lines 33 of Fig. 2.

Fig. 4 is a sectional view of one of the holding means for the speaker mounting member.

Essentially in the practice of my invention, I

attach a loudspeaker to a supplementary mounting means which is resiliently mounted on the main mounting means. In this way transmission of the vibrations to the cabinet itself directly is cut out, and the tendency to microphonism becomes wholly inconsiderable.

I provide a means for interrupting and diffusing the beam of high frequencies from the center of the cone, to accomplish other of the objects set forth above; and the remainder of the difficulties which I have mentioned are obviated essentially by the provision of acoustic loading for the loudspeaker in a way which I shall describe in detail.

Referring to the figures, I have indicated at I a radio cabinet, having a lower portion 2, in which the loudspeaker 5 is mounted. The front portion of the cabinet indicated at 6 (see especially Fig. 3), is cut away at 1, over a general area of the cone of the speaker. It need not of course, be entirely cut away. It is usual and desirable to form a sort of grill in the front panel of the radio cabinet by cutting away portions in an ornamental design. The grill on the reverse side may be covered with an ornamental cloth or the like m 8, as is also quite usual. I have shown a main board or baffle 9, held solidly to the front panel of the cabinet by means of screws H) or the like, usually with the interposition of the cloth 8 therebetween. As I have previously indicated however, the use of this main board 9 is dictated primarily as a matter of convenience in cabinet construction. The front panels of the cabinets are frequently not strong enough to support adequately a heavy loudspeaker. The board rigidifies the front of the cabinet, andalso contributes to the strength of the cabinet, and prevents racking. However, in cabinets having strength enough as initially constructed, the mounting board 9 may be eliminated.

Where used, it will be cut out centrally as at II. It has hitherto been the practice to cut out a baifie board over something less than the full area of the cone. Where however, my acoustic loading is employed, as I shall describe hereinafter, it is very desirable to depart from the former procedure and cut out the board over an area at least as large as the area of the cone of the speaker in order to avoid attenuation of the sound. It goes without saying of course, that the same thing is true of the front panel of the cabinet. Unless the front panel of the cabinet is made of material which has substantially no sound interrupting qualities, the openings in it should be effective over an area at least as large as the area of the cone.

Instead of mounting my speaker directly to the board 9, I mount it to a special mounting member indicated at [2. This also is a board. It is resiliently mounted on the board 9 and a particular way of doing this which I have found advantageous, is illustrated in Fig. 4. Here a bolt i3 passes through the board 9 and is held by a nut l4 bearing against a washer l5, the shank of the bolt extending therebeyond. A heavy washer I6 is next placed on the bolt followed by a rubber sleeve 51. The mounting means [2 is perforated to accept the rubber sleeve, and is placed thereon as shown. Next another heavy washer I 3 is placed on the bolt shank. The washers l6 and 18 may conveniently be made of heavy fiber, Bakelite, or the like, though they may also be made of metal, especially where the sleeve i7 is long enough to prevent any contact between these washers and the board l2. A smaller washer l9 and nut 29 may complete the assembly. When the nut 23 is tightened the effect will be to compress the rubber sleeve ll, causing it to expand inside the perforation in the board I2, and grip this board, so as to fix its position in a resilient way.

The general mounting assembly indicated in Fig. 4, is marked in the other views with the index numeral 2|.

The mounting board l2 may be, and preferably will be, of ornamental shape. I have shown one having the shape of a lyre. It may be made of a single piece of wood or other substance, and I have found ply wood to be an advantageous material. The board does not need to be acoustically dead, since it is resiliently mounted. It should be thick enough to be rigid under the particular circumstances of its use, and in practice I make the board thicker where it is intended for the mounting of larger and heavier speakers, as will be within the skill of the man in the art to determine for himself. The perforations which I have shown in the board at 22, are merely a part of the ornamental configuration of it. I have found it advisable in view of the exigencies of shipment of radio cabinets to fasten to the rear side of the board [2, some rubber bumpers 2-3. These have only a contingent function in preventing displacement and damage to the mounting means in transit, and normally have no acoustic effect. The board, as I have indicated, is resiliently mounted to the cabinet or to a mounting board 9 by means of the assemblies 2!.

The loudspeaker 5 is fastened directly to the board 52 by means of bolts 24, or equivalent suitable fastening means.

Instead of employing the board i2 as the usual baflie, and cutting it out over a portion at least of the effective area of the cone, I treat the board in a special way to accomplish an acoustical loading and the diffusion of the beam of high frequencies to which I have referred above. I do this by arranging a series of perforations in the board. A central, and preferably circular portion of the board, indicated at 25, is left solid, to interrupt the central beam of high frequencies. Around this there are arranged a series of perforations 26, to let the high frequencies through in a diffused manner. The central portion 25 is maintained in position by arms 27, left when forming the perforations 26. The external effective diameter of the perforations 26 is very substantially less than the effective diameter of the cone, which I have indicated by the dotted line 28 in Fig. 2. This leaves in front of the exterior portions of the cone, a substantial mass 23 of the panel or mounting means l2. This mass is perforated with a series of relatively small holes 30, preferably spaced at regular intervals, though they may be otherwise spaced if circumstances require it. The purpose of this is to provide acoustic loading for the speaker. The cone of the speaker has to pump air in and out through these perforations as it operates to reproduce the sound. I have found that this loads the cone in such a way as not only to destroy the wolf tones which I have mentioned, but also to do away to a considerable extent with natural resonances in the speaker itself.

The size and placement of the holes 30 and the area of the mounting member I2 in which they are placed, are matters which are best determined empirically for each class or type of speaker which it is proposed to employ. In the light of my teachings however, the skilled Worker in the art will readily be able empirically to determine a degree of acoustic loading which gives the best results with any particular size and type of speaker; and I have found that once having determined this for any given speaker, identical mounting means will work with entire satisfaction for all speakers of the same class, type and size. As a general rule, the amount of acoustical loading will be roughly proportional to the size of the speaker cone. The variation in the diameter of the beam of high frequencies sent out by the central portion of a loudspeaker, is not usually sufficient to require much alteration of the size of the perforations 26, and the central solid portion 25. As the size of speaker cones increases, therefore the area 29 will increase, and for the larger speakers, more rows of the perforations 30 will usually be provided. It will be understood that the shape and disposition of the various perforations may be varied without departing from the spirit of my invention, and that the perforations, having due regard to the purposes of my invention, may be arranged in various ornamental designs as desired.

I have found that the arrangement of parts which I have described gives greatly superior reproduction for reasons which will be entirely clear, it is believed, from the discussion of the problems involved in the early part of these specifications. Radio sets equipped with loud speakers mounted as herein taught, have no tendency toward micophonism, give a diffused sound not characterized by clear auditory placement, unequal volume in space, nor disadvantageous nodal characteristics. Wolf tones have been eliminated and resonance peaks greatly diminished. The reproduction of sound is more natural and much more pleasant.

My method of speaker loading is especially applicable to circuits employing an output tube or tubes of high plate impedance characteristics. When using such tubes, for example pentode tubes, the speaker is very lightly loaded because of the tube load. The normally light loading of the speaker when such tubes are used is the fundamental reason for the tendency to employ triode output tubes where high quality reproduction is desired in the modern radio set. But the invention herein described operates to supply the speaker loading which tubes such as triodes with low plate impedance would automatically furnish; and therefore the way is opened up to the use of high plate impedance output tubes in radio sets which must have high quality reproduction.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. In combination with a loudspeaker having a cone, a mounting panel against which said loudspeaker is attached, said panel extending across the face of the said cone so as to cover at least the major portion of the area of said cone, said covering portion being perforated at intervals with a series of spaced perforations of a size to provide acoustic loading for said cone, a central portion of said covering portion being solid and located in the path of the central beam of high frequencies reproduced by said cone, said central portion being bordered by cut-away portions of said panel of suflicient size to permit diffused passage of said high frequencies without substantial acoustic loading for said high frequencies.

2. A mounting panel for a cone type loudspeaker characterized by a central imperforate portion lying in the path of the central beam of high frequencies from said speaker, an intermediate portion characterized by perforations of sufficient size to permit diffused passage of said high frequencies without substantial acoustic loading, and an outlying portion having a series of relatively small, and relatively numerous, spaced perforations to provide acoustic loading for peripheral portions of the cone of said speak- 3. The device of claim 2 in combination with resilient mounting means.

4. The mounting panel of claim 2 in combination with a base panel and resilient means for holding said panels together, said base panel being cut away over the face of the cone to permit the passage of sound without acoustic loading, said cut-away portions having an effective size at least as great as the effective area of the cone of said loudspeaker.

EMMERY J. H. BUSSARD. 

